Should I be wary if my boyfriend talks to a girl(who hooks up with all his friends), then trys hides it?

He texts her quite frequently. Just the other night he told me he was going to hang out with the guys, then he got a text from her asking him if we was gonna go hang out with her afterall that night. I had no idea that he had her number until he went over his text messages(he never texts me), so I checked the bill and he texts her SOOO much. I don't think that they have done anything YET, but I am not sure if I should even stick around to find out. This girl has hooked up with at least 2 of his friends already. When I am around her, she is flirty with him, and asks mean to me. I am so lost on what to do. He is the type of guy who has a lot of girl friends, and I don't want to tell him he can't talk to all girls, but this is not the first girl that he has hid from me. I tell him to communicate with me about it and I won't be so upset, but for some reason he feels he must hide it from me. WHAT SHOULD I DO? Am I wrong for feeling that there is something going on here? I need help!